The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Automatic transmissions typically include several fluid operated torque transmitting devices such as clutches. The clutches automatically engage and disengage according to a predefined pattern to establish different speed ratios between input and output shafts of the transmission.
The various speed ratios of the transmission are typically defined in terms of the ratio Ni/No, where Ni is the input shaft speed and No is the output shaft speed. Shifting between various speed ratios generally involves disengaging a clutch associated with the current or actual speed ratio, and engaging a clutch associated with the desired speed ratio. The clutch to be released is referred to as the off-going clutch, while the clutch to be engaged is referred to as the on-coming clutch. Shifts of this type are referred to as clutch-to-clutch because no speed responsive or freewheeling elements are used.
The engaging and releasing of the clutches is controlled by solenoid operated valves that supply hydraulic fluid to the clutches. For example, a variable bleed solenoid (VBS) valve is a current-controlled, electro-hydraulic actuator that provides an outlet pressure that is a precise function of the current applied to the valve. A constant pressure is supplied to the valve through a fixed control orifice to a control chamber formed by the valve. The control chamber pressure can be controlled by allowing the control chamber to bleed to a reservoir through the variable orifice formed by the VBS valve. With this structure, a VBS valve can regulate fluid pressure from a maximum value to a minimum value.